This is the second revision of a grant requesting funds to continue a highly productive 5 year grant to study how viruses cause autoimmune insulin-dependent diabetes mellitus (IDDM). Utilizing recent information we derived from a transgenic (tg) model created to mimic human IDDM together with reports of other investigators a unifying hypothesis of how viruses cause IDDM and other autoimmune diseases is presented. The testing of this hypothesis comprises this grant proposal. We hypothesize for the initial event to occur the virus must be both tropic for the target organ and via infection of the target organ induce cytokines/chemokines in the local milieu allowing upregulation of MHC and T cell activation molecules, APC and attraction of antiself T cells. When the unresponsive but potential autoimmune antigen-specific T cell population in the periphery is of sufficient numbers and affinity then the virus through stimulation of the innate immune system can cause disease. However, when the antigen-specific antiself T cell pool in the periphery is of low numbers and of low affinity (as often in most instances of autoimmune disease) then the virus must provide a mimic, through the process we initially termed molecular mimicry, so that the T cell pool is sufficiently expanded (adoptive immune system) to cause disease. This grant tests the components of this hypothesis with studies of the role of virus tropism, the role of selected components of the innate and adoptive immune response and determines numbers of antigen-specific anti "self" T cells deposited in the target organ in situ in comparison to non-antigen-specific T cells in causing IDDM. To do these experiments we utilize both our established RIP-LCMV and MBP-LCMV tg mice, create new tg models that specifically inhibit viral replication in the target organ, use recombinant viruses tropic or not tropic for pancreas or CNS that carry the "autoimmune self" "viral" antigen, and use MHC tetramers and functional assays to identify and quantitate the antiself antigen-specific T cells deposited in the target organ as well as following their trafficking to that tissue.